Various roller coating methods and apparatus exist to apply solutions to moving flexible webs. One such method includes a two-roll coating process where an application roller is partially immersed and rotated in a hopper containing coating solution. A portion of the application roller rising out of the hopper containing coating solution is brought into controlled close contact with a substrate which is wrapped around another roller placed above the application roller. The space between the application roller and the substrate is typically between 0.003 inches and 0.015 inches. A bead of coating solution is formed in this space or gap, a portion of which is deposited on the moving substrate. The remainder of the coating solution remains on the roll as a thin liquid coating. The coating solution is supplied to the hopper through an inlet and the level of the solution in the hopper is controlled by a suitable control device.
Flow visualization studies have shown that the solution in the hopper stagnates due to turbulent eddy currents set up in the hopper by a combination of the rotating application roller and the flow of coating solution into the hopper. As a result of this stagnation, particulates that enter the hopper remain in the hopper and are sporadically deposited on the moving web causing unacceptable defects and waste. Particulates can enter the hopper through various vectors. These studies also show that when the solution is drained from the opposite end of the hopper, the eddy currents do not change, particulates are not purged and the flow is stratified with the particles remaining at the liquid/application roller interface continuing to generate coating defects.
Thus, there exists a need to reduce these turbulent eddies within the liquid coating hopper. The present invention minimizes eddies within a hopper containing coating solution and an application roller thereby improving coating quality and minimizing waste.